The present invention generally relates to multistage recrystallization, and more particularly to a multistage method and apparatus particularly well-suited for running under closed conditions to minimize the influence of environmental variations on the method and apparatus. Crystallizable substances are purified to extremely high levels, typically on the order of 99.999 percent purity. Materials are transferred within and between stages of the multistage method and apparatus in a manner whereby no crystallized material need be moved between vessels or stages.
Purification of crystallizable materials by way of recrystallization techniques has been known and practiced for many years. Generally speaking, it is extremely difficult to move trace impurities from crystallizable materials by recrystallization from a saturated solution therefor, or by other techniques including fractional crystallization and fractional freezing procedures. Included are zone refining procedures which typically would increase product purity by adding recrystallization stages, but this would be done at the expense of the percentage yield of product that is formed at the last or purest recrystallization stage. For example, it is well known to use fractional crystallization in separating radium chloride from barium chloride by a classic separation procedure that utilizes a complicated grid pattern of many dozens of separate evaporation dishes or crystallizers. Under this procedure, the crystal crop of each crystallizer is transferred to the adjacent crystallizer in one direction, while the mother liquor is transferred in the opposite, or countercurrent direction. Although increasing concentrations of radium chloride follow movement of the crystal crop from one crystallizer to the next, a very large number of crystallizers are required, and operating costs for this type of system are very high.
Generally speaking, currently available applications for fractional crystallization have recognized the desirability of operating at an optimum reflux ratio while not necessarily effectively providing suitable specifics for implementing this desirable approach. It can be important that reflux ratio conditions be controlled closely, preferably in association with the establishment and maintenance of steady state conditions in each stage of the system, thereby facilitating generally automatic control of reflux ratio conditions by closely monitoring and metering selected material transfers, inputs and/or outputs for each stage of the system. Included as examples in this regard are Griffiths U.S. Pat. Nos. 4,885,061 and 5,127,921, which are incorporated by reference hereinto. Practices of this type typically require the transfer of solid, crystallized material between vessels and between stages of these multistage recrystallization systems.
The multistage recrystallization art also includes multistep recrystallization systems for separating fluid material having two or more different components. For example, U.S. Pat. No. Re. 32,241 describes having a component crystallize on a cooled surface as material containing the component flows down the cooled surface. With this approach, a solution of a given stage is used to wash the crystals formed in that stage before the crystals are transferred to the next purer stage. This wash solution is the solution that is in this stage at the time immediately preceding the crystallization of any material therefrom. In an embodiment of this development, the crystals are transferred to the next purer stage as soon as appropriate valves cause the liquid of the next purer stage to contact and dissolve the crystals. The wash solution, which is used to wash the crystal crop of the given stage, is the purest part of the solution present in that given stage before any material is crystallized from it.
With the approach of U.S. Pat. No. Re. 32,241, the stages preferably operate sequentially. During crystallization, a self-supporting solid layer of crystal cake is formed on the chilled surface of the crystallizer after the mother liquor is drained. This system is not particularly well-suited for refining crystallizable substances which cannot form the self-supporting, adherent layer of solid crystal cake on the chilled surface of the crystallizer. This system is not particularly useful for feed materials which form a flowable slurry or any other form that does not adhere to a chilled surface, such as substances which are recrystallized from a solution comprised of a solute of the material being refined and a solvent for this material. Additionally, crystal cakes formed by systems of this type are not particularly well-suited for effective washing of individual crystals without breaking up the cake and then washing the pulverized crystals with a suitable wash solution. Typically, these cakes are hard and rigid, and only crystals on its exposed surface are effectively washed, the interior crystals not being readily reached by the wash solution.
U.S. Pat. No. 4,787,985 suggests multistage purification through use of a sequential recrystallizer wherein the size and purity of the crystals increases from stage to stage. Crystals are processed by the use of thickener devices or wash columns, and this approach has the disadvantage that crystals are not dissolved and then recrystallized from a purer solution than that from which they were originally formed.
U.S. Pat. No. 4,588,414 suggests the use of a system including a centrifuge arrangement. This countercurrent approach embodies the use of temperature differences between components, with crystals being removed from one crystallizer and being fed to another crystallizer at a higher temperature while the mother liquor moves to another crystallizer of lower temperature.
It has been found that, by proceeding in accordance with the present invention, it is possible to achieve superpurification of crystallizable substances by removal of certain impurities therefrom through a multistage recrystallization procedure that achieves purity levels on the order of 99.999 percent with extremely high yields that are exceptional for multistage recrystallization procedures. Included is an effective manner of controlling reflux ratio conditions while achieving and maintaining steady state operating conditions.